A liposome is a closed vesicle having a lipid bilayer structure. A liposome can encapsulate drugs or genes in a state isolated from the external environment with a bimolecular membrane, and thus can protect the encapsulated drugs or genes from being decomposed or metabolized. In addition, a liposome can be attached to a cell membrane and mucous membrane by controlling the composition of the liposomal membrane, and thus it is possible to deliver the encapsulated drugs or genes into cells. Liposomes are attracting attention as a carrier for drugs or genes because of such protective function and delivery function.
In general, drugs and genes used for treatment of diseases are desired to be delivered to a target site of action and to exert an intended pharmacological action at such a site. Application of liposomes is attempted to improve delivery characteristics for drugs or genes to the target site of action. Means to control the kinds, ratio and surface charges of constituent lipids have been proposed to give liposomes a selective transport function to the target site of action. However, the above prior art methods cannot adequately control delivery characteristics for drugs under present circumstances. Particularly, drugs and genes applied to lung tissue are required to highly control in vivo behavior of drugs or genes depending on its mode of action, i.e., retention on the surface of lung tissue (e.g., on the bronchovesicular surface) is desired in some cases and incorporation inside lung tissue is desired in other cases. However, techniques to control in vivo behavior of drugs and genes in lung tissue are not established yet.
Meanwhile, techniques to modify the surface of liposomes with macromolecules such as polymers have been reported (e.g., Non-Patent Documents 1 and 2). However, techniques to control in vivo behavior of drugs or genes in lung tissue by modifying the surface of liposomes are not sufficiently known.